1. Field of the Invention
This invention relates to the manufacture of electronic components. More specifically, this invention relates to the manufacture of semiconductor components, and in particular to the preparation of semiconductor wafers prior to high temperature processing steps, such as diffusion, ion implantation, epitaxial growth, and chemical vapor deposition steps, and especially to methods and apparatus for the prediffusion cleaning of semiconductor wafers.
2. Brief Description of the Prior Art
In the fabrication of semiconductor wafers several proces steps require contacting the wafers with fluids. Examples of such process steps include etching, photoresist stripping, and prediffusion cleaning. Often the chemicals utilized in these steps are quite dangerous in that they may be strong acids, alkalis, or volatile solvents.
The equipment conventionally used for contacting semiconductor wafers generally consists of a series of tanks or sinks into which racks of semiconductor wafers are dipped. Wafer carriers are described, for example, in U.S. Pat. Nos. 3,607,478, 3,964,957 and 3,977,926. Such conventional wet processing apparatus poses several difficulties.
Since the tanks are open to the atmosphere, airborne particulates can enter into the process solutions. Through surface tension these particles are easily transferred to the wafer surfaces as the wafers are dipped into and lifted out of the sinks. This particulate contamination is extremely detrimental to the microscopic circuits which the wafer fabrication process creates. It is especially important to minimize particulate contamination during prediffusion cleaning.
Also, since the process chemical tanks are open, operators are exposed to the dangerous chemicals. This is a recognized safety concern. Furthermore, the chemicals must be regularly replaced with new solutions and this is generally accomplished by aspirating the solution out of the sink and pouring fresh solution from a bottle into the sink. Clean room operators usually wear face masks, gauntlet gloves, and chemical aprons to prevent injury during the manual replacement of process fluids.
Process disadvantages are also apparent for the sinktype method. As the chemicals age, they become contaminated with impurities from the air and from the wafers. The treatment of the last batch of wafers prior to fluid rejuvenation may not be as effective as treatment of the first batch of wafers in a new solution. Non-uniform treatment is a major concern of semiconductor process engineers.
Conventional wet process station sinks are characterized by stagnant conditions. Chemical process reactions occur at the surface of the wafer and stagnant conditions yield boundary-layer or "filming" phenomena which result in certain areas of the wafer receiving less chemical action than others. This is undesirable and could be particularly crucial in an etching operation. Good temperature control may also suffer because of stagnant conditions. Wet process equipment manufacturers have attempted to incorporate agitating means to circumvent these problems. One method attempting to overcome non-uniform etching is disclosed in U.S. Pat. No. 3,437,543.
After fluid processing the wafers normally need to be dried. This can be a particularly challenging process because it is important that no contamination be created during the drying process. Evaporation is undesirable since it often leads to spotting or streaking. Even the evaporation of ultra high purity water can lead to problems because such water is very aggressive to the wafers' surfaces and will dissolve traces of silicon and silicon dioxide. Subsequent evaporation will leave residues of the solute material on the wafer surface. Contamination and other causes of semiconductor failure are discussed, for example, in J. Schadel, "Device Failure Mechanisms In Integrated Circuits," Solid State Devices 1983 Conf. Ser. No. 69 (Institute of Physics, London 1984) 105-120.
A device know as a spin-rinser-drier is often used to dry wafers without water evaporation. These machines utilize centrifugal force to "throw" the water off the wafer surfaces. Problems can arise with wafer breakage because of the mechanical stress placed on the wafers, particularly with larger wafer sizes. Manufacturers are also challenged to minimize particulate generation in these usually somewhat complicated mechanical devices. Notwithstanding this, some manufacturers have extended the spin-rinser-drier technology to general fluid processing.
Machines known as "acid processors" are now available which spray the process fluids onto wafer surfaces. These machines offer the advantage of conserving chemical reagents while at the same time permitting each batch of wafers to be exposed to fresh solutions. Just as the shower was an advance over taking a bath, the acid process is an advance over the sink method. Yet several difficulties have prevented acid processors from gaining widespread use.
The difficulties with spin-rinser-driers in terms of particle generation and mechanical complexity are compounded in acid processors. Instead of just high purity water, acid processors must deal with the aggressiveness of hot corrosive acid used in fluid processing. The problems of mechanial breakdown and particulate generation are not ameliorated by use of acid processors instead of spin-rinser-driers.
Spray methods also introduce new process difficulties. Because only a small amount of reagent is utilized, it is nearly impossible to achieve high temperature processing. In prediffusion cleaning, for example, it is desirable to expose the wafers to concentrated sulfuric acid at a temperature between 80 to 150 degrees centigrade. Because the heat capacity of the sprayed solution is not very great, especially relative to the wafers themselves, it is nearly impossible to achieve the desired temperature on the wafer surface. The uniformity and control of temperature in acid processors is questionable. Similarly, it is difficult to achieve uniformity and control of processes such as hydrofluoric acid etching in which a relatively fast chemical reaction is taking place.
Even safety concerns are not obviated by enclosed acid processors. The spray nozzles of these machines use relatively high chemical pressures to achieve the correct spray pattern. Anytime such dangerous chemicals are pressurized, there is the possibility of an accident.
What is needed in the area of semiconductor wet processing is an apparatus and process which can bring process chemicals into contact with semiconductor wafers in such a manner that: (a) atmospheric contamination is avoided, (b) personnel exposure to the chemicals is minimized, (c) uniform treatment of the wafer is promoted, (d) fast and effective heat transfer is allowed, (e) stagnant conditions are minimized and "filming" effects are avoided, (f) the machine is mechanically simple, and (g) aggressive chemicals are handled safely.